The development of industrially acceptable catalytic anodes involves complex problems since the industrial performance of an anode depends on numerous intricately linked and interacting factors such as for example: the choice of anode materials, its manufacturing process and conditions, the industrial application and operating conditions of the anode.
While a multitude of patents reflect on one hand the great interest and considerable efforts to develop catalytic anodes, it is quite remarkable that only very few anode embodiments are on the other hand industrially applied on a large scale. This striking discrepancy between innumerable anodes proposed and the very few embodiments actually applied in practice is nevertheless not so surprising in light of the complex problems and interacting factors generally indicated above which make any attempt to develop an anode that fully meets the extremely severe technical and economic industrial requirements a particularly difficult and unpredictable undertaking.
Lead or lead alloy anodes have been widely used in processes for electrowinning metals from sulphate solutions. They nevertheless have important limitations, such as a high oxygen overvoltage and loss of the anode material leading to contamination of the electrolyte, as well as the metal product obtained on the cathode. Anodes of lead-silver alloy provide a certain decrease of the oxygen overvoltage and improvement of the current efficiency, but they still have the said limitations as a whole.
In order to ensure uniform electrodeposition of metal on the cathode, metal electrowwining cells are generally operated with a current density of a few hundred A/m.sup.2 and consequently require a very large anode surface area, so that the anode cost with respect to the relatively low value of the metal electrowon per unit area is critical and must be kept sufficiently low to be economically justified for electrowinning.
It has been proposed to use dimensionally stable titanium anodes with a catalytic oxide coating for oxygen evolution. Such oxygen anodes are nevertheless subject to extremely severe oxidation attack and corrosion due to anodically evolved oxygen and exposure to highly corrosive electrolytes, which leads to anode passivation due to loss of the anode coating and oxidation of the titanium base. It has moreover been proposed to provide the titanium base of such anodes with a protective undercoating comprising a platinum group metal, before applying the outer catalytic coating. However, these coatings generally provide insufficient protection of the titanium base to justify the high cost of using such dimensionally stable titanium anodes comprising precious metals.
Dimensionally stable anodes with mixed oxide coatings comprising platinum group metals and valve metals are described in U.S. Pat. No. 3,632,498. An example of this patent relates to the preparation of a fine Ti-Pd mixed oxide powder which is then applied by rolling or hammering into a rod of soft-quality titanium. However, the amount of precious metal incorporated in the mixed oxide powder and applied to the electrode in this manner could be prohibitive for various industrial applications. Thus, when the electrode surface is to be substantially covered with the mixed oxide powder, and more particularly when the electrode is intended for operation at a relatively low current density such as is used in metal electrowinning, the cost of precious metal thus applied in the form of a mixed oxide may be especially prohibitive.
Ruthenium was for the first time successfully applied by H. Beer to produce dimensionally stable anodes with a mixed oxide coating which combines the high stability of rutile TiO.sub.2 with the excellent electrocatalytic properties of RuO.sub.2 for chlorine evolution, while raising the oxygen potential, and thereby enhancing the selectivity for producing chlorine as opposed to oxygen. These significant advantages and more particularly the high stability of such anodes explain their outstanding success in the chlorine industry throughout the world since more than 15 years.
On the other hand, such an anode with a TiO.sub.2 -RuO.sub.2 mixed oxide coating hardly seems suitable for use as an oxygen evolving anode since it neither exhibits a sufficiently low oxygen potential, nor sufficiently protects the titanium base against oxygen evolution. Such an anode thus does not provide sufficient energy savings, nor an adequate service life to justify the anode costs for most applications of oxygen evolving anodes.
According to Example 8 of U.S. Pat. No. 4,052,271, an anode having a titanium core provided with a coating comprising iridium oxide and manganese oxide, is stated to have a chemical resistance far superior to that of a core coated with metallic iridium, and to be particularly suited for the preparation of per-compounds.
U.S. Pat. No. 4,289,591 relates to a method of electrolytically generating oxygen which comprises providing a catalytic cathode and a catalytic oxygen evolving anode respectively bonded to opposite surfaces of a solid polymer electrolyte ion transporting membrane, a catalyst being provided at the anode comprising ruthenium oxide and manganese oxide. Accordng to this patent, said catalyst is produced by a modified Adams method, wherein ruthenium and manganese salts are mixed, an excess of sodium nitrate is incorporated, the mixture is fused at 500.degree. C. for three hours, and the residue is washed and dried to provide ruthenium oxide-manganese oxide powder, which is then bonded to said solid polymer electrolyte ion transport membrane. However, the described modified Adams method for producing a finely divided catalyst powder is not intended, nor seems particularly suitable for manufacturing a complete electrode structure.
Oxygen evolving anodes with a manganese oxide coating on a titanium core have likewise been proposed, but hardly provide a sufficient catalytic effect, significant energy saving, or an adequate long-term service life to justify their cost in industrial applications.
A catalytic lead-based oxygen evolving anode recently invented by H. Beer is described in his previous application Ser. No. 293,384 filed Aug. 17, 1981 (now U.S. Pat. No. 4,425,217), which is hereby incorporated by reference herein.
The catalytic lead-based anode according to said recent invention of H. Beer, essentially comprises catalytic particles of valve metal activated with a minor amount of platinum group metal, which are uniformly distributed on and partly embedded in the surface of an anode base of lead or lead alloy; said catalytic particles are thereby firmly anchored and electrically connected to said anode base, while their remaining non-embedded part projects from said surface of the anode base, thereby presenting a larger projecting surface than the underlying surface of the anode base of lead or lead alloy. Oxygen can thereby be catalytically evolved on said projecting surface of the partly embedded catalytic particles at a reduced potential at which the underlying lead or lead alloy remains electrochemically inactive and thereby essentially serves as a current-conducting support for said partly embedded catalytic particles.
Such a catalytic, lead-based oxygen evolving anode can provide various advantages, which may be summarized as follows:
(a) It can be operated at a significantly reduced oxygen potential, well below that of conventional anodes of lead or lead alloy currently used in industrial metal electrowinning, whereby the energy costs for electrowinning metals may be decreased accordingly. PA0 (b) Contamination of the electrolyte and the cathodic deposit can be substantially avoided, since it has been established that the lead or lead alloy of the anode base is effectively protected from corrosion when oxygen is evolved on the catalytic particles at such reduced potentials. PA0 (c) Dendrite formation on the cathode will lead to no serious deterioration of the performance of this anode, since it operates at a reduced potential, at which lead base which does not undergo notable corrosion. PA0 (d) Conventional lead or lead alloy anodes may be readily converted into such a catalytic anode, so that industrial electrowinning cells may be retrofitted simply and inexpensively to provide improved performance. PA0 (e) The reduced cell voltage obtained with such a catalytic anode can be readily monitored to rapidly detect any notable rise of the anode potential, so that spent catalytic particles may be readily replaced or rectivated. PA0 (f) Ruthenium can be used very economically by applying it in minimal amounts (less than 6 wt%) to a many times larger amount of titanium sponge particles partly embedded in the anode base, so that the precious metal cost may be justified by the improvement in anode performance. Restricted amounts of ruthenium can thus be advantageously combined with less expensive materials. PA0 (g) Decreased short-circuits could be observed in copper electrowinning plants equipped with such catalytic anodes, which would result in improved cathodic current efficiency, and thereby further increase the energy savings due to anode operation at a reduced oxygen potential.
The industrial development of said catalytic lead-based anode, more particularly as a catalytic oxygen anode suitable for metal electrowinning, has shown that ruthenium must be employed to catalytically activate the valve metal particles, since the use of other precious metals, which are far more expensive and much less available, could be prohibitive, and may be impractical.
Ruthenium is a relatively inexpensive precious metal which is capable of providing excellent catalytic activity for oxygen evolution, but it is on the other hand known that ruthenium by itself, in metallic form or as a simple oxide, lacks adequate stability under oxygen-evolving conditions, this being due to its tendency, at anode potentials above 1.43 V vs. NHE, to form RuO.sub.4 which is highly volatile, and hence completely unstable.
It is nevertheless necessary to maintain high, long-term catalytic activity with a low oxygen potential for a sufficiently extended period to justify the costs of using such a catalytic anode by the total energy savings it provides during its useful service life.
In the course of said industrial development, it was moreover found in this connection that the main advantages underlying such a catalytic lead-based oxyben as described above could be achieved ore fully, while the useful anode service life could be considerably extended, if a relatively large amount of ruthenium (up to 20 g/m.sup.2, or more) is applied to a correspondingly increased amount of titanium sponge particles (e.g. 700 to 800 g/m.sup.2), partly embedded in the lead anode base.
However, for this purpose, the ruthenium had to be stabilized in a suitable manner, and this important technical problem underlies the present invention.